Processes are known for preparing olefin polymers and copolymers in the presence of highly active catalysts obtained by reacting an organometallic Al compound with a Ti compound supported on a Mg dihalide, and in which an electron donor compound is combined with the Ti compound and/or the Mg compound and, optionally, also with the Al compound.
The catalysts employed in these processes permit the obtaining of such high polymer yields with respect to the Ti compound contained in the catalyst, as to avoid need for purifying the polymer from the catalyst residues.
In some applications, in particular in the alimentary and medico-sanitary fields, however, it is desirable to reduce to the absolute minimum possible the amount of said residues, in particular of the halogenated compounds and organic substances.
British Pat. No. 1,420,837 describes a method which results in a remarkable reduction of the halogen content in polyolefins prepared with conventional catalysts not supported on Mg dihalies, by treating the polymer with gaseous mixtures consisting of an olefinic oxide, in particular propylene oxide, nitrogen and vapor in appropriate ratios.
U.S. Pat. No. 4,029,877 describes a process for purifying polymers prepared with catalysts optionally comprising also a Mg dihalide as a support, such process consisting in treating the polyolefin powder with a weak-reactive gas consisting of a mixture of an inert gas with oxygen or vapor or the mixtures thereof, in which the oxygen concentration ranges from 0.05 to 0.5 and the vapor from 0.05 to 1% by moles. Said treatment results in improvement in the color characteristics of the polymer.
Processes capable of reducing the content of both halogenated substances and organic compounds in the olefin polymers prepared with catalysts supported on a Mg dihalide and comprising an electron-donor compound as catalyst modifier, have not been known heretofore.